Main Bearings - Chevrolet 1980 Light Duty Truck Service Manual

Fig. 6A4-40~Gaging Plastic On Crankpin
Inspection and Replacement
1. With oil pan and oil pump removed, remove the
connecting rod cap and bearing.
Before removal of connecting rod cap, mark the side
of the rod and cap with the cylinder number to assure
matched reassembly of rod and cap.
2. Inspect the bearing for evidence of wear or damage.
(Bearings showing the above should not be installed.)
3. Wipe both upper and lower bearing shells and
crankpin clean of oil.
4. Measure the crankpin for out-of-round or taper
with a micrometer. If not within specifications replace or
recondition the crankshaft. If within specifications and a
new bearing is to be installed, measure the maximum
diameter of the crankpin to determine new bearing size
required.
5. If within specifications measure new or used bearing
clearances with Plastigage or its equivalent.
If a bearing is being fitted to an out-of-round crankpin,
be sure to fit to the maximum diameter of the crankpin. If
the bearing is fitted to the minimum diameter and the
crankpin is out-of-round .001" interference between the
bearing and crankpin will result in rapid bearing failure.
a. Place a piece of gaging plastic, the length of the
bearing (parallel to the crankshaft), on the crankpin or
bearing surface (fig. 6A4-40). Plastic gage should be
positioned in the middle of upper or lower bearing shell.
(Bearings are eccentric and false readings could occur if
placed elsewhere).
b. Install the bearing in the connecting rod and cap.
c. Install the bearing cap and evenly torque nuts to
specifications.
Do not turn the crankshaft with the gaging plastic
installed.
Fig. 6A4-4 1--Measuring Gaging Plastic
. d. Remove the bearing cap and using the scale on the
gaging plastic envelope, measure the gaging plastic width
at the widest point (fig.6A4-41).
6. If the clearance exceeds specifications, select a new,
correct size, bearing and remeasure the clearance.
Be sure to check what size bearing is being removed
in order to determine proper replacement size bearing. If
clearance cannot be brought to within specifications, the
crankpin will have to be ground undersize. If the crankpin
is already at maximum undersize, replace crankshaft.
7. Coat the bearing surface with oil, install the rod cap
and torque nuts to specifications.
8. When all connecting rod bearings have been
installed tap each rod lightly (parallel to the crankpin) to
make sure they have clearance.
9. Measure all connecting rod side clearances (see
specifications) between connecting rod caps (fig. 6A4-42).
M A IN BEARINGS
Main bearings are of the precision insert type and do
not utilize shims for adjustment. If clearances are found to
be excessive, a new bearing, both upper and lower halves,
will be required. Service bearings are available in standard
size and .001", .002", .009", .010" and .020" undersize.
Selective fitting of both rod and main bearing inserts
is necessary in production in order to obtain close
tolerances. For this reason you may find one half of a
standard insert with one half of a .001" undersize insert
which will decrease the clearance .0005" from using a full
standard bearing.
When a production crankshaft cannot be precision
fitted by this method, it is then ground .009" undersize ON
ONLY THOSE M A IN JOURNALS TH A T C A N N O T
BE PROPERLY FITTED. ALL JOURNALS W ILL
NOT NECESSARILY BE GROUND.
bearing or .010" undersize bearing will then be used for
precision fitting in the same manner as previously described.
S M ALL BLOCK 6 A 4 -23
A .009" undersize
LIG H T T R U C K S ER V ICE M ANUAL